This invention relates to a speed control apparatus for an elevator, and more particularly to an elevator speed control apparatus of the type wherein an inverter is employed for the speed control of a hoisting induction motor and wherein the inverter is controlled by such processing means as a microcomputer.
Heretofore, a speed control apparatus for an elevator shown in FIG. 1 has been proposed as one of the type wherein an inverter is employed for the speed controller of a hoisting induction motor and wherein the inverter is controlled by processing means (as which a microcomputer shall be exemplified hereinafter). Referring to FIG. 1, numeral 1 designates a three-phase A.C. power supply, numeral 2 a converter which converts three-phase alternating current into direct current, numeral 3 a smoothing capacitor which is connected to the D.C. side of the converter 2 and smooths the D.C. output, and numeral 4 an inverter which is connected across the terminals of the smoothing capacitor 3 and inverts the direct current into alternating current of variable voltage and variable frequency in accordance with the pulse width modulation system (hereinbelow, abbreviated to `PWM`) and which is constructed of transistors and diodes. Numeral 5 indicates a current detector which detects the output current of the inverter 4, and numeral 6 a three-phase induction motor which is driven by the A.C. output of the inverter 4 and to which a speed detector, e.g., tachometer generator 7 and a sheave 8 are directly coupled. Shown at numeral 9 is a traction rope which is wound round the sheave 8, and which has the cage 10 of the elevator coupled to one end thereof and the counterweight 11 coupled to the other end thereof.
Numeral 12 indicates a pattern generator which generates the speed pattern of the elevator, and numeral 13 a microcomputer which is fed with the output signal 7a of the speed detector 7 at a predetermined time interval, which operates it in comparison with the output signal 12a of the speed pattern generator 12 and which controls the inverter 4 with the operated output. This microcomputer 13 is composed of a central processing unit (CPU) 16 as well as interface circuits 14 and 15 for feeding the output signals of the speed pattern generator 12 and the speed detector 7 to the central processing unit 16 respectively, a memory device 17 in the form of a RAM and a memory device 18 in the form of a ROM for exchanging data with the central processing unit 16, and interface circuits 19 and 20 for sending a current command generating circuit 21 data etc. processed by the central processing unit 16. In addition, numeral 22 indicates a pulse width modulation circuit (hereinbelow, termed `PWM circuit`) which compares an output signal from the current command generating circuit 21 with the output signal of the current detector 5 and applies pulse width modulation, and numeral 23 a base drive circuit which amplifies the output signal of the PWM circuit 22 so as to apply base signals to the transistors of the inverter 4.
FIG. 2 is a block diagram showing the details of the current command generating circuit 21 in FIG. 1. This circuit comprises a D/A converter 24 which converts into an analog quantity a current command value (digital quantity) 19a sent from the microcomputer 13 through the interface circuit 19, an oscillator 25 which converts into a train of pulses a frequency command 20a sent from the microcomputer 13 through the interface circuit 20, as well as a counter 26 which counts the output pulses of the oscillator 25, and memory circuits 27, 28 and 29 which provide data of sinusoidal waves of different phases and corresponding to phase U, V and W in accordance with the count value of the counter 26, respectively. The output data of the respective memory circuits 27-29 are delivered to corresponding D/A converters 30, 31 and 32. The respective D/A converters 30-32 convert the output data of the memory devices 27-29 into analog quantities with the current command value of the D/A converter 24 as a reference value, to send the current command values of the U, V and W phases.
FIG. 3 shows a functional block diagram expressive of a speed control calculation unit owing to the microcomputer 13. The output signal 7a of the speed detector 7 is subtracted from the pattern signal 12a of the speed pattern generator 12 by a subtracter 131, and the output signal of the difference is subjected to a PI control operation by a PI control calculation portion 132 so as to provide a torque command (T.sub.L) 132a. A current calculation portion 133 calculates the current command value I on the basis of the torque command 132a in accordance with Eq. (1) mentioned below, and the current command value I is output as indicated by symbol 19a through the interface circuit 19 (FIG. 1). ##EQU1## where I.sub.o : exciting current,
K.sub.T : coefficient of a torque current, which is determined depending upon the motor.
A slip frequency calculation portion 134 calculates a slip frequency .omega..sub.s on the basis of the torque command T.sub.L in accordance with the following equation (2) : EQU .omega..sub.s =K.sub.s .multidot.T.sub.L ( 2)
where K.sub.s : coefficient of a slip frequency, which is determined depending upon the motor.
The output signal obtained by the calculation in the slip frequency calculation portion 134 is added with the output signal 7a of the speed detector 7 in an adder 135, and the result is output as the frequency command 20a.
FIGS. 4(a) and 4(b) illustrate the relationship between the speed and slip frequency of the induction motor during the acceleration of the induction motor. A solid line 40 in FIG. 4(a) represents the actual rotational frequency of the induction motor, a broken line 41 the output signal of the speed detector 7 which is loaded into the microcomputer 13 through the interface circuit 15 at a fixed period, and a dot-and-dash line 42 the output status of the frequency command value 20a.
Meanwhile, when the operations are executed at fixed cycles by the speed control calculation unit shown in FIG. 3, the actual slip frequency fails to agree with the computed value as the rotational frequency of the induction motor changes. This situation is depicted in FIG. 4(b).
In FIG. 4(b), a solid line 43 represents the output status of the slip frequency calculation portion 134, and a dot-and-dash line 44 the actual slip frequency. As apparent from FIG. 4(b), the actual slip frequency (average value) becomes smaller than the output of the slip frequency calculation portion 134. This produces the overvoltage of the induction motor. In the mode of regenerative braking, the actual slip frequency becomes greater relative to the above, resulting in an increase of the current of the induction motor.